There has been known an HUD device that is mounted in a vehicle and virtually displays an image to be viewable by an occupant. An HUD device disclosed in Patent Literature 1 projects an image on a projection member located above an instrument panel in a vehicle. The HUD device includes a projector that projects an image as light polarized in a predetermined polarization direction and a reflecting mirror that reflects and redirects the light of the image from the projector toward the projection member, on an optical path between the projector and the projection member.
The reflecting mirror is formed by adhering a polarizing film to one surface of a light-transmissive plate-shaped substrate. A reflection axis thereof is set in accordance with the polarization axis of the projector, that is, so as to form an angle of 45° with respect to the horizontal direction of the image. Thus, part of external light such as sunlight that enters the reflecting mirror is cut.